Scattering cell employing electrostatic means for supporting a particle

ABSTRACT

A levitator for use with a light scattering photometer unit including a spaced pair of plate electrodes to provide an electric field for producing a first electrostatic force on a charged particle located between the spaced plate electrodes and with the levitator additionally including a pin electrode extending through and insulated from one of the plate electrodes to provide an electric field for producing a second electrostatic force and with the combination of the first and second electrostatic forces suspending the charged particle between the plate electrodes at a location spaced from but adjacent to the pin electrode. An automatic servo system includes an optical detector for detecting the position of the charged particle to produce a control signal to adjust the electric fields to maintain the charged particle in the proper position.

United States Patent [1 Phillips et al.

[ SCATTERING CELL EMPLOYING ELECTROSTATIC MEANS FOR SUPPORTING APARTICLE [75] Inventors: David T. Phillips; Herman II.

Brooks, both of Goleta, Calif.; Philip J. Wyatt; Chelcie R. Lin, both ofSanta Barbara, Calif.

[73] Assignee: Science Spectrum, Inc., Santa Barbara, Calif.

22 Filed: on. 21, 1971 [21] Appl. No.: 191,373

OTHER PUBLICATIONS Applied Optics Vol. 9, pages 2522-2528; Blau, Jr.,McCleese, Watson. Journal of Colloid Science Vol. 16, pgs. 68-84, Gucker& Egan.

' [451 Aug. 28, 1973 Physical Review Vol. 32, pg. 349, first series,R.A. Millikan.

Physical Review Vol. 4, pg. 440 second series, Fletcher.

Primary Examiner-Ronald L. Wibert Assistant Examiner-Conrad ClarkAttorney-Charles H. Schwartz et al.

[57] ABSTRACT A levitator for use with a light scattering photometerunit including a spaced pair of plate electrodes to provide an electricfield for producing a first electrostatic force on a charged particlelocated between the spaced plate electrodes and with the levitatoradditionally including a pin electrode extending through and insulatedfrom one of the plate electrodes to provide an electric field forproducing a second electrostatic force and with the combination of thefirst and second electrostatic forces suspending the charged particlebetween the plate electrodes at a location spaced from but adjacent tothe pin electrode. An automatic servo system includes an opticaldetector for detecting the position of the charged particle to produce acontrol signal to adjust the electric fields to maintain the chargedparticle in the proper position.

14 Claims, 8 Drawing Figures Patented Aug. 28, 1973 3,754,830

4 Sheets-Sheet 2 Patented Aug. 28, 1973 3,754,830

4 Sheets-Sheet s SCATTERING CELL EMPLOYKNG ELECTROSTATIC MEANS FORSUPPORTING A PARTICLE The present invention is directed to a levitatorfor use with a light scattering photometer unit. As one specificexample, the light scattering photometer unit may operate in thefollowing manner. Microparticles, whose light scattering properties areto be investigated, are located in a beam of light produced by a lightsource, preferably the light energy produced by the light source ispolarized and at a single frequency, such as the light energy producedby various lasers. The microparticles may be of many types, such asbacteria or latex spheres of about 1 micrometer in diameter, but thelevitator of the present invention could also be used with particles ofgreater or lesser size. The light scattered by the microparticle isintercepted by a detector mounted beneath a periscope which moves in anare around the particle. The signal from the detector is amplified todrive a recorder to record a plot of the scattered light intensity as afunction of the angle of the detector relative to the incident beam oflight, this plot being a differential light scattering pattern.

A clearer understanding of the operation of a light scatteringphotometer may be had with reference to U.S. Pat. application Ser. No.777,837, filed on Nov. 21, 1968, in the name of Philip J. Wyatt now US.Pat. No. 3,624,835, and Ser. No. 34,243, filed on May 4, 1970, in thenames of Philip J. Wyatt, et al., and both assigned to the same assigneeas the instant application.

In the present invention, a stream of microparticles whose lightscattering properties are to be investigated is introduced into ascattering chamber such as by nebulizing a liquid suspension or by anyother convenient means. Most microparticles upon being nebulized from aliquid suspension or when collected from an aerosol source naturallytend to have a small positive or-nega tive charge. An individualmicroparticle is isolated and positioned in the center of a laser beamthrough the use of pneumatic and electrical controls. Once the particleis suspended in the laser beam, the particle may be automatically heldin position by means of an automatic servo mechanism so that the lightscattering properties of the particle may be investigated.

Generally, the particle is maintained in the proper position through theuse of a levitator which includes a pair of parallel plate membersserving as electrodes and with a third pin electrode extending throughbut insulated from one of the parallel plate electrodes. The combinationof the three electrodes provides for electric fields to maintain theparticle suspended between the parallel plates and in a position forinterception with the light beam.

In the prior art it is known to use a pair of parallel plate electrodesto suspend a particle between the electrodes. For example, Millikan, inthe early part of this century, used parallel plate electrodes tosuspend small oil drops between the plates. Generally, with the priorart apparatus, the plates were charged to provide an electric fieldinteracting with the charge of the particles to counterbalance the forceof gravity to maintain the particle between the plates. The difficultywith this type of apparatus is that the particle would not necessarilybe centered and would tend to move off from between the plates. Inaddition, the electric field would have to be very accurately maintainedand parallel to the gravitational field which required that the platesbe very accurately aligned-and perpendicular to the local gravitationalfield or the particle would tend to move within the space between theplates. These above difficulties made the use of the Millikan-typeapparatus impractical other than for demonstrations.

In order to overcome some of the difficulties of the Millikan apparatus,Fletcher, in 1914, added a third electrode which took the form of asmall plate located within and insulated from one of the parallelplates, which third electrode was used to produce an electric field topull the particle in toward the center, and also to counterbalance thegravitational forces on the particle. By using this center electrode atselected times to produce an electric field between the center electrodeand the pair of parallel plate electrodes, the particle could bereturned to the center position. The difficulty with the Fletcherapparatus is thatthe size of the radial field produced by the smallthird electrode and used for centering is limited by the fact that, inaddition to producing the radial field, the third electrode alsoproduces a vertical field. The vertical field is limited to a level tocounterbalance gravity which, in turn, limits the size of the radialfield. The Fletcher apparatus, therefore, provides for a very weakradial field which is often not effective.

ln orderto overcome the shortcomings of both the Millikan and Fletcherapparatus, the present invention uses a pair of spaced parallel plateelectrodes and with a pin electrode extending through and insulated fromone of the plate electrodes. In a preferred embodiment of the inventionthe pin electrode extends through the top electrode and wherein the twoparallel plate electrodes are charged to provide an electric field toproduce a force which is down to add to the gravitational force on theparticle. in order to compensate for this electric field produced by theparallel plate electrodes, the center pin electrode is charged relativeto each of the plate electrodes to provide a field to produce a forcewhich is up" to overcome both the effects of gravity and the electricfield pushing down thereby balancing the particle between the plates.This up" field is therefore relatively large since it must overcome boththe down field and gravity.

In addition to the up" field produced by the pin electrode, a radialfield is also produced, which is quite large since it is produced by thesame charge as the up field. This combination of fields provides astrong radial field to maintain the particle in a centered positionwhile the pair of fields, one up" and one down," is used to overcome theeffects of gravity to maintain the particle between the plates. Once theparticle is in the center position, a servo system with an opticaldetector may adjust the magnitude of the fields to keep the particlesuspended in the proper position.

The above arrangement of two parallel plates forming a pair ofelectrodes and with a third electrode extending through and insulatedfrom one of the parallel plates so as to provide for a strong radialfield while still balancing the particle may be energized in a number ofdifferent ways. For example, the electrodes may be energized alternatelyso that the up" and in" field can be produced alternately with the down"field to produce the strong net radial field. The alternation must berapid relative to any rate of motion of the particle. in addition, bothfields may be produced simultaneously so as to produce the strong netradial field while still providing the balancing of the particle. Theinvention will be described with reference to an arrangement wherein thefields are produced simultaneously, but it is to be appreciated that thefields could be applied alternately, as explained above.

A clearer understanding of the invention will be had with reference tothe following description and drawings, wherein:

FIG. la, 11;, and 1c illustrate in schematic form the operation of thethree-electrode levitator of the present invention;

FIG. 2 illustrates in detail a particular construction for a scatteringcell including a levitator for receiving and suspending themicroparticles;

FIG. 3 illustrates a general arrangement of the scattering cell 1 ofFIG. 2 in combination with the various pneumatic controls forintroducing microparticles in the cell;

FIG. 4 illustrates a view of the control panel used to provide theelectrical and pneumatic control of the microparticles within thescattering cell of FIG. 2;

FIG. 5 illustrates the arrangement of the optical detector and otheroptics for use with scattering cell of FIG. 2; and

FIG. 6 illustrates a block diagram of a servo for controlling theposition of the microparticles within the scattering cell.

Referring first to FIGS. la, lb, and la, a schematic representation ofthe levitator of the present invention is shown. Specifically, thelevitator includes a pair of parallel plate electrodes 10 and 12 and athird pin electrode 14 extending through but insulated from theelectrode 10. As shown in FIG. 1c, the various electrodes may havepotentials applied thereto, the values of which, shown in FIG. 10, arerepresentative only. For a given particle, the electrode 10 may have areference potential or be at ground, the electrode 12 may have apotential on the order of +50 volts, and the electrode 14 may have apotential on the order of +100 volts. These positive potentials for theelectrical energy applied to the electrodes 12 and 14 are based on anassumption that the microparticle to be suspended has a negative chargeas shown by charged particle 16 located in a suspended position. If theparticle had a positive charge, instead, then the corresponding voltageson electrodes 10, l2 and 14 would be reversed.

Referring now to FIG. la, it can be seen that the difference inpotential between the electrodes 12 and l4 provides for strong electricfield producing a force which is up and in" as shown by arrows 18. Thedifference in potential between the electrodes 10 and 14 provides forstrong electric field producing a force which is radial and in," asshown by the arrows 20. If the electrode 12 had no potential, anegatively charged particle would therefore be drawn toward the centerpin electrode 14.

Referring now to FIG. lb, the difference in potential between theelectrodes 10 and 12 provides for weak electric field producing a forcewhich is down, as shown by the arrows 22, and if no potential wereapplied to. electrode 14, a negatively charged particle would be pulleddownward toward the electrode 12.

' Since the particle also is pulled downward by the force or gravity,the electric field as shown by the arrows 22 is in a direction toincrease this downward movement. FIG. 10 shows the combination of these.electric fields. Specifically, the strong radial electric field isstill present to hold the particle 16 in a center position. A combinedvertical field 24 is a combination of the fields l8 and 22 shown inFIGS. 1a and 1b and is weaker than the field shown by the arrows 20.This weak vertical field is used to counterbalance the force of gravityof the charged particle 16. It can be seen, 7

therefore, that by the use of this three-electrode structure, andspecifically in the particular manner in which this three-electrodestructure is energized, the gravitational forces acting on the chargedparticles may be counterbalanced by a weak vertical field, while at thesame time a strong radial field is used to pull the charged particleinto a central position.

It is to be appreciated that the pin electrode 14 may extend through thebottom electrode instead of the top electrode and with the electricfields arranged to provide for the charged particle properly positionedbetween the electrodes. Also, the plane of the plate electrodes need notbe perpendicular to the force of gravity and the plate electrode may bedisposed in any angular relationship to the force of gravity and withthe charged particle still held suspended between the plate electrodes.

FIG. 2 is an exploded view of a scattering cell including a levitator ofthe present invention to receive microparticles and to provide for thedetection of the differential light scattering properties of theseparticles. Since air currents may exert a substantial force on asuspended particle, preferably all parts of the scattering cell arepneumatically sealed to one another, as by 0- rings, and the inlet andoutlet also include means permitting them to be selectivelypneumatically sealed from exterior pressure fluctuations. The scatteringcell includes a cover member 100, having an inlet connector 102 and aflush connector 104. The cover fits over a settling chamber 106 whichreceives the microparticles. The cover and settling chamber are sealedby an O-ring 107. The base 10 of the settling chamber 106 is the firstupper electrode 10 shown in FIG. 1. The pin electrode 14 extends throughan insulating plug 108 which is received in an opening 110 in the base10.

An electrical connector 112 is mounted on the outside wall of thesettling chamber 106 to provide for the application of electricalpotential through the wire 114 to the pin electrode 14. The plateelectrode 10 and the settling chamber 106 are grounded through theelectrical connector 112. A bellows connector 116 extends through thewall of the settling chamber 106 An opening 118 in the insulator 108allows microparticles to pass from the settling chamber106 into thelight scattering area of the scattering cell of FIG. 2.

The light scattering area is formed by a transparent cell member 120which includes a masked section 122. Light energy, such as from a laserbeam, passes into the transparent cell 120 through an entrance mask 124.A light trap 126 receives the light energy after it passes throughthetransparent cell. A light'exit 128 may be provided through the maskedportion 122 so that the interior of thecell may be visually observedwith a microscope in a manner to be described in a later portion of thisspecification. This sealed light exit is wedged at a slight angle to theaxis of the scattering cell in the preferred embodiment to avoidreflecting light back into thecell in the horizontal plane in whichscattered light is viewed. The settling chamber 106 and the transparentcell 120 are sealed by an upper O-ring 130.

The light ene gy, when appropriately directed through the transparentcell 120 from the entrance port 124, intersects any microparticlelocated at the proper position in the transparent cell and provides fordifferential light scattering in accordance with the differential lightscatteringproperties of scattering properties microparticle. Thisdifferential light scattering may be detected by viewing thedifferentially scattered light at different angular positions throughthe transparent portion of the transparent cell 120.

A base member 132 supports the lower plate electrode 12. The base member132 and the transparent cell 120 are sealed using a lower O-ring 134. Anopening 136 extends through the lower plate electrode and is connectedto an exhaust connector 138 to provide the exhausting of microparticleswithin the transparent cell 120. Finally, an electrical connection forapplying potential to the lower plate electrode 12 is provided byelectrical connector 140 which may be an opening to receive a plug, suchas a banana plug.

FIG. 3 illustrates a typical manner in which the scattering cell of FIG.2 may be interconnected with other elements to introduce microparticlesto the transparent cell. Specifically, a nebulizer 200 may be used toprovide individual microparticles to the scattering cell through a hose202 connected to the inlet connector 102. The nebulizer 200 operates ina known manner using a supply of a filtered gas, such as air to atomizeportions of a liquid suspension of microparticles and provide a streamof the gas containing individual microparticles. As another example,microparticles may be introduced by drawing an aerosol of microparticlesinto a syringe, thus connecting the outlet of the syringe to line 202and selectively injecting the entrained microparticles by collapsing thesyringe.

When a nebulizer is employed, a supply of air is coupled through a hose204 and is connected to an air control valve 206. The air control valve206 has two outputs, one of which is through a hose member 208 directlyto the flush connector 104. An air control switch 210 controls theapplication of air from the hose 204 through the hose 208 to flush outthe scattering cell. A second output from the air control valve 206 isthrough a hose 210 to a nebulizer control valve 212. The output from thenebulizer control valve is through a hose member 214 which is connectedto the nebulizer 200. A nebulizer button control member 216 is used tocontrol the application of air to the nebulizer 200. When the button 216is pushed, air is supplied to the nebulizer 200 to provide for theintroduction of microparticles contained in the stream of air throughthe hose 202 to the inlet connector 102.

A bellows assembly '218 is used to supply a gentle supply of air througha hose member 220 to the bellows connector 116. Finally, an exhaust hose222 is connectcd to the exhaust connector 138 to provide for an exhaustof any microparticles within the scattering cell. The interior of thescattering cell is pneumatically isolated from any external air pressurechanges and the flow through the scattering cell is controlled by thebellows assembly 218 and the nebulizer 200.

FIG. 4 illustrates the control panel of the levitator ofautomatic-manual-position knob 230, a bellows control knob 232 and afocus control knob 234. Also, an eyepiece 236 of a microscope extendsfrom the control panel.

The use of the various control knobs shown on the control panel of FIG.4'are as follows:

The air control switch 210 in its forward position provides for theflushing of the scattering cell with a clean gas, such as filtered air.In the rear position of the switch 210, the air control valve 206 shownin FIG. 3 is connected to have the air supplied to the nebulizer controlvalve 212. The nebulizer control button 216, when pushed, supplies airto the nebulizer to spray particles from the nebulizer into thescattering cell. The polarity knob 224 controls the electrode voltagesto be either plus or minus so that particles of either charge polaritycan be levitated within the scattering cell.

The voltage knob 226 sets the maximum voltage which may be applied tothe scattering cell electrodes. The balance knob 228 is set to balancethe input to the servo system for the desired position of the particle.The position knob 230 may be in one of two modes, depending upon whetherthe knob is pulled out or whether the knob is pushed in. When the knob230 is pulled out, the levitator is in the manual mode. Rotating theknob 230 adjusts the voltages provided to the electrodes so as to movethe particle within the scattering cell. When the knob is .pushed in,the levitator is in the automatic mode, and the levitator servo controlsthe electrode voltages to maintain the particle in the proper positionwithin the scattering cell.

The bellows control knob 232 is used to flex a small bellows to move airand particles slowly through the scattering cell. Finally, the focuscontrol knob 234 is used to focus the microscope so as to visuallyobserve the center of the scattering cell.

FIG. 5 illustrates the optical arrangement for visually observing thecenter of the scattering cell and for providing for an optical detectionof the position of the particle within the scattering cell. The lightenergy scattered by the particle near the center of the scattering cell,as shown by arrows 300, leaves the scattering cell through the exit 128and is focused by a lens structure 302 onto a beam splitter 304. Aportion of the light energy passes through the beam splitter 304 to adiagonal mirror 306. A second portion of the light energy is reflectedby the beam splitter 304 to a viewing mirror 310 and may be viewedthrough a microscope 308. When the eyepiece 236 of the microscope 308 isproperly focused, a visual observation may be had of the central regionof the scattering cell.

The light energy which impinges on the diagonal mirror 306 is directedupward toward a second diagonal mirror 312. A portion of the lightenergy is reflected by the second diagonal mirror 312 to a firstphotomultiplier 314. In addition, a portion of the light energy from thediagonal mirror 306 passes by the second diagonal mirror 312 to impingeon a second photomultiplier 316. The combination of the twophotomultipliers 314 and 316 with the second diagonal mirror 312 may bei used to provide an optical detection of the vertical position of theparticle within the scattering cell. For example, if there are noparticles in the scattering cell in the path of the laser beam, then nolight energy would be scattered out of the exit 128 and no light energywould therefore pass'to the diagonal mirror 306. If there is a particlein the path of the light beam within the scattering cell, then theposition of this particle determines the intensity distribution of thescattered light passing through the exit which in turn determines theamount of light energy which is received by each of .the twophotomultipliers 314 and 316. In this way, the vertical position of theparticle determines the relative output of the photomultipliers 314 and316.

FIG. 6 illustrates the operation of the automatic servoing of thelevitator so as to maintain the microparticle in the proper positionwithin the scattering cell. In FIG. 6 the three-electrode levitator ofthe present invention, including the parallel plate electrodes 10 and 12and the center pin electrode 14 is shown with the plate electrode 10grounded and with voltages applied to the electrodes 12 and 14. A laserbeam supplies light energy to the scattering cell and a scatteredportion of the light energy is directed toward the mirror 312. Thescattered light strikes the edge of the mirror 312 which divides thisscattered light between the two photomultiplier tubes 314 and 316. Theintensity of the light received by the two photomultiplier tubes 314 and316 is equal when the particle is in the proper position within thescattering cell.

The output from the photomultipliers 314 and 316 are applied to a pairof matched characteristic logarithmic input amplifiers 350 and 352,which logarithmic amplifiers convert the photomultiplier output currentto a voltage proportional to the logarithm of the current from thephotomultiplier. The use of logarithmic amplifiers maintains constantservo gain and provides servo stability for a wide range of particles.The balance control 228 is adjusted to provide equal gain for equalsignals into the input amplifiers 350 and 352. The outputs from theamplifiers 350 and 352 are applied to a difference circuit 354 and thedifference between the two voltages is amplified by isolation amplifier356 and applied to a sample and hold circuit 358. The output of thesample and hold circuit 358 is controlled by a oneshot multivibrator 360which controls a switch 362. Since a pulsed laser is used in a preferredembodiment, the output signals from the input amplifiers 350 and 352 areused to control the multivibrator 360. If a continuous wave laser isused, the sample and hold circuit may be ommitted, or replaced by anaveraging circuit.

The position control 230(a) which is the resistive control portion ofthe positioning control switch 230 shown in FIG. 4 is used to control aninput signal to an isolation amplifier 364. In addition, the positioncontrol switch 230 may be either in a manual or an automatic mode ascontrolled by the switch portion 230(b) of the position control. Themanual mode is when the switch 230(b) is closed, which controls themultivibrator 360 to maintain the switch 362 in an open position. Atthis time only the position control 230( a) effects the voltages appliedto the electrodes of the levitator. When the switch 230(b) is in theopen position, the servoing is automatic. At this time input signals areprovided from the differencing circuit 354 to the sample and holdcircuit 358 so that the output of the sample and hold circuit is anerror signal in accordance with the position of the particle.

As indicated above, the output from the sample and hold circuit 358 isapplied to the isolation amplifier 364 and then to a pair of d-c to d-cconverters 366 and 368.

The maximum output from the d-c to d-c converters 366 and 368 isadjusted by the voltage control 226, but below that maximum value theoutput from the d-c to d-c converters are control signals which areapplied through the polarity switch 224 to the electrodes 12 and 14 tomaintain the particle in the proper position within the scattering cell.

The operation of the levitator in isolating and positioning anindividual particle within the laser beam with the structure shown inFIGS. 2-5 is as follows:

First, the nebulizer 200 is filled with a suspension of i themicroparticles to be studied and the hoses are connected to thescattering cell in the manner shown in FIG. 3. The various electricalpower requirements are coupled to the instrument so that the laser (orother light source) is energized and the electrodes in the levitator arealso energized. Initially the polarity control 224 may be positioned tothe plus position. The voltage control 226 is adjusted to zero voltswhich is normally in the full counter-clockwise position. The positioncontrol 230 is in the manual mode and turned counterclockwise whichnormally means that the particle is pulled downward.

In order to clear the scattering cell, the air toggle switch 210 isswitched forward to flush the scattering cell with clean air. This maybe visually observed by watching through the eyepiece 236 of themicroscope 308 until no particles can be seen passing through the laserbeam. When this has occurred, toggle switch 210 is returned to the rearposition to connect up the nebulizer 200. The nebulizer button 216 maythen be pushed briefly a few times until several particles are observedpassing through the laser beam with each pulse of the nebulizer button216. Now the bellows knob 232 may be moved back and forth to moveparticles slowly through the beam. The larger particles may be easilyseen and for smaller particles the defraction images of the particlesappear as sharply defined concentric rings will be visible through themicroscope. The focus knob 234 may be adjusted for maximum imagesharpness. When there is a bright image in the field of view of themicroscope, representing a particle, the voltage control 226 may beturned up until the image can be observed moving in response to thevoltage control 226. The motion of the particle should be in thedownward direction. If the motion is not downward, this means that thepolarity of the particle is reversed and the polarity control 224 shouldbe switched to the minus position. The position knob 230 may be moved toarrest the downward motion of the particle and bring the particle backtoward the center of the field of view of the microscope. Actually, boththe voltage and position controls may be simultaneously manipulated soas to stop the particle's motion before it leaves the region withinwhich it can be seen.

If the particle is lost, then the voltage and position controls may bereturned to zero and the nebulizer button pushed again to bring newparticles in the field of view. Since the particles may have differentpolarities, it is important to remember that the polarity control mayhave to be reversed in order to provide the downward motion. As analternative to the above, it may be more convenient to begin with thepolarity control 224 at the zero position and with the voltage control226 set high enough to produce relatively fast motion. The positioningof the particle may then be accomplished by simultaneously manipulatingthe polarity and position controls instead of the voltage and positioncontrols.

When a particle has been positioned in the center of the field of viewand remains nearly stationary, then the position control 230 is pushedin to place the levitator in the automatic mode. This engages theautomatic servo control shown in FIG. 6 so as to hold the particle fixedwithin the light beam. The voltage control 226 may be turned completelyup so as to provide a maximum automatic servoing of the position of theparticle within the levitator.

The present invention therefore provides a simple structure forpositioning a microparticle within a beam of light, such as light from alaser, and maintaining that particle automatically in this position. Thelevitator includes a pair of parallel plate electrodes and a thirdelectrode extending through and insulated from one of the plateelectrodes. In a preferred embodiment of the invention, the pair ofparallel plate electrodes provide an electric field to pull the particledown to aid the force of gravity and with the center electrode providingan electric field to pull the particle up and counterbalance thedownward forces and also to pull the particle toward the center. Thisstructure allows for a relatively strong electric field to be used topull the particle into the central position. The maintaining of theposition of the particle may be automatically controlled using anoptical detector in combination with a servo system.

The invention has been described with reference to 5. The levitator ofclaim 1 additionally including means detecting the position of a chargedparticle located between the electrodes and for providing an errorsignal representing the difference between the actual position of thecharged particle and a desired position for the charged particle and foradjusting the potential applied to the electrodes to adjust the positiona particular embodiment but the invention is only to be 1 limited by theappended claims.

We claim:

1. A scattering cell for receiving charged particles for intersectionwith a beam of light and including a levitator for suspending chargedparticles within the scattering cell, including a pair of parallel plateelectrodes and an insulating wall member forming an included space andincluding an entrance area for a beam of light,

a third pin electrode extending through and insulated from one of theparallel plate electrodes,

first means for introducing charged particles into the enclosed space,

second means for providing a beam of light through the entrance area ofthe wall member and past a position adjacent to the third pin electrode,

third means for supplying an electrical potential between the pair ofparallel plate electrodes to produce a first electric field in a firstdirection, and fourth means for supplying an electrical potentialbetween the pin electrode and the pair of parallel plate electrodes toproduce a second electric field in a second direction opposite to thedirection of the first electric field to form a composite field betweenthe pair of plate electrodes to balance forces operating on any chargedparticles located between the pair of plate electrodes and to produce athird radial electric field to pull any charged particles locatedbetween the pair of plate electrodes toward a position adjacent the pinelectrode.

2. The scattering cell of claim I wherein the first means forintroducing charged particles into the enclosed space includes anebulizer. v

3. The scattering cell of claim 1 additionally including a bellowsassembly for pneumatically controlling the movement of the chargedparticles in the enclosed space. i i

4. The scattering cell of claim 1 additionally includ ing controls foradjusting the magnitude and polarity of the electrical potentialsapplied to the electrodes.

of the charged particle.

6. The levitator of claim l wherein the enclosed space is pneumaticallyisolated from any external changes in pressure.

7. A scattering cell for receiving microparticles and including alevitator for suspending a microparticle in a fixed location to beintercepted by a beam of light, in-

cluding an electrode structure including a pair of electrodes spacedfrom each other and supplied with an electrical potential between thepair of electrodes to produce a first electric field on a microparticlelocated between the pair of electrodes and including a third pinelectrode extending through and insulated from one of the first pair ofelectrodes and -supplied with an electrical potential between the thirdelectrode and the pair of electrodes to produce a second electric fieldto oppose the first electrical field on the microparticle locatedbetween the pair of electrodes to balance the microparticle and toproduce a radial electric field to pull the microparticle to a positionadjacent to and on the axis of the third pin electrode, first means forintroducing microparticles between the pair of electrodes, and secondmeans for producing a beam of light to pass through the positionadjacent to the third pin electrode to intercept microparticles at suchposition. 8. The scattering cell of claim 7 wherein the first means forintroducing microparticles between the pair of electrodes includes anebulizer.

9. The scattering cell of claim 7 additionally including a bellowsassembly for controlling the movement of the microparticles between thepair of electrodes.

10. The scattering cell of claim 7 additionally including controls foradjusting the magnitude and polarity of the electrical potentialsapplied to the electrodes.

11. The levitator of claim 7 additionally including means detecting theposition of a microparticle located between the pair of electrodes andfor providing an error signal representing the difference between theactual position of the microparticle and a desired position for themicroparticle and for adjusting the potential applied to the electrodesto adjust the position of the microparticle.

12. The levitator of claim 7 wherein the electrodes structure forms anenclosed space and wherein the enclosed space is pneumatically isolatedfrom an changes in external pressure.

l3. A scattering cell, for receiving charged particles for intersectionwith a beam of light and including a levitator for suspending chargedparticles within the scattering cell, including a pair of parallel plateelectrodes and an insulating wall member forming an included space andincluding an entrance area for a beam of light, a detection area forobserving scattered light at different angular positions relative to thecharged particles, and a beveled port for observing a portion of thescattered light,

a third pin electrode extending through and insulated from one of thepairs of parallel plate electrodes,

first means for introducing charged particles into the enclosed space,

second means for providing a beam of light through the entrance area ofthe wall member and past a position adjacent to the third pin electrode,

third means for supplying an electrical potential between the pair ofparallel plate electrodes to produce a first electric field in a firstdirection,

fourth means for supplying an electrical potential between the pinelectrode and the pair of parallel plate electrodes to produce a secondelectric field in a second direction opposite to the direction of thefirst electric field to form a composite field between the pair of plateelectrodes to balance forces operating on any charged particles locatedbetween the pair of plate electrodes and to produce a third radialelectric field to pull any charged particles located between the pair ofplate electrodes toward a position adjacent the pin electrode, and

fifth means for detecting the portion of the scattered light passingthrough the beveled port and for providing an error signal representingthe difference between the actual position of the charged particle and adesired position for the charged particle and for adjusting thepotential applied to the electrodes to adjust the position of thecharged particle.

14. The scattering cell of claim 13 wherein the fifth means includes abeam splitter for directing the scattered light to a pair of lightdetecting devices and with the difference between the output from thelight detecting devices providing the error signal.

i t t

1. A scattering cell for receiving charged particles for intersection with a beam of light and including a levitator for suspending charged particles within the scattering celL, including a pair of parallel plate electrodes and an insulating wall member forming an included space and including an entrance area for a beam of light, a third pin electrode extending through and insulated from one of the parallel plate electrodes, first means for introducing charged particles into the enclosed space, second means for providing a beam of light through the entrance area of the wall member and past a position adjacent to the third pin electrode, third means for supplying an electrical potential between the pair of parallel plate electrodes to produce a first electric field in a first direction, and fourth means for supplying an electrical potential between the pin electrode and the pair of parallel plate electrodes to produce a second electric field in a second direction opposite to the direction of the first electric field to form a composite field between the pair of plate electrodes to balance forces operating on any charged particles located between the pair of plate electrodes and to produce a third radial electric field to pull any charged particles located between the pair of plate electrodes toward a position adjacent the pin electrode.
 2. The scattering cell of claim 1 wherein the first means for introducing charged particles into the enclosed space includes a nebulizer.
 3. The scattering cell of claim 1 additionally including a bellows assembly for pneumatically controlling the movement of the charged particles in the enclosed space.
 4. The scattering cell of claim 1 additionally including controls for adjusting the magnitude and polarity of the electrical potentials applied to the electrodes.
 5. The levitator of claim 1 additionally including means detecting the position of a charged particle located between the electrodes and for providing an error signal representing the difference between the actual position of the charged particle and a desired position for the charged particle and for adjusting the potential applied to the electrodes to adjust the position of the charged particle.
 6. The levitator of claim 1 wherein the enclosed space is pneumatically isolated from any external changes in pressure.
 7. A scattering cell for receiving microparticles and including a levitator for suspending a microparticle in a fixed location to be intercepted by a beam of light, including an electrode structure including a pair of electrodes spaced from each other and supplied with an electrical potential between the pair of electrodes to produce a first electric field on a microparticle located between the pair of electrodes and including a third pin electrode extending through and insulated from one of the first pair of electrodes and supplied with an electrical potential between the third electrode and the pair of electrodes to produce a second electric field to oppose the first electrical field on the microparticle located between the pair of electrodes to balance the microparticle and to produce a radial electric field to pull the microparticle to a position adjacent to and on the axis of the third pin electrode, first means for introducing microparticles between the pair of electrodes, and second means for producing a beam of light to pass through the position adjacent to the third pin electrode to intercept microparticles at such position.
 8. The scattering cell of claim 7 wherein the first means for introducing microparticles between the pair of electrodes includes a nebulizer.
 9. The scattering cell of claim 7 additionally including a bellows assembly for controlling the movement of the microparticles between the pair of electrodes.
 10. The scattering cell of claim 7 additionally including controls for adjusting the magnitude and polarity of the electrical potentials applied to the electrodes.
 11. The levitator of claim 7 additionally including means detecting the position of a microparticle located between the pair of electrodes and for providing an error signal representing the difference between the actual position of the microparticle and a desired position for the microparticle and for adjusting the potential applied to the electrodes to adjust the position of the microparticle.
 12. The levitator of claim 7 wherein the electrodes structure forms an enclosed space and wherein the enclosed space is pneumatically isolated from any changes in external pressure.
 13. A scattering cell for receiving charged particles for intersection with a beam of light and including a levitator for suspending charged particles within the scattering cell, including a pair of parallel plate electrodes and an insulating wall member forming an included space and including an entrance area for a beam of light, a detection area for observing scattered light at different angular positions relative to the charged particles, and a beveled port for observing a portion of the scattered light, a third pin electrode extending through and insulated from one of the pairs of parallel plate electrodes, first means for introducing charged particles into the enclosed space, second means for providing a beam of light through the entrance area of the wall member and past a position adjacent to the third pin electrode, third means for supplying an electrical potential between the pair of parallel plate electrodes to produce a first electric field in a first direction, fourth means for supplying an electrical potential between the pin electrode and the pair of parallel plate electrodes to produce a second electric field in a second direction opposite to the direction of the first electric field to form a composite field between the pair of plate electrodes to balance forces operating on any charged particles located between the pair of plate electrodes and to produce a third radial electric field to pull any charged particles located between the pair of plate electrodes toward a position adjacent the pin electrode, and fifth means for detecting the portion of the scattered light passing through the beveled port and for providing an error signal representing the difference between the actual position of the charged particle and a desired position for the charged particle and for adjusting the potential applied to the electrodes to adjust the position of the charged particle.
 14. The scattering cell of claim 13 wherein the fifth means includes a beam splitter for directing the scattered light to a pair of light detecting devices and with the difference between the output from the light detecting devices providing the error signal. 